The Prophet ﷺ said, "There will come a people who hold a covenant of protection. Whoever kills one of them will not catch the fragrance of Paradise, though its scent can be detected from a distance of seventy years' travel."
قال حدثنا أبو النضر قال حدثنا الأشجعي عن سفيان عن الأعمش عن هلال بن يساف
عن النبي ﷺ أنه قال سيكون قوم لهم عهد فمن قتل رجلا منهم لم يرح رائحة الجنة وإن ريحها ليوجد من مسيرة سبعين عاما
Its isnad is sahih. Its narrators are leading authorities, well known, and Hilal bin Yasaf, or Isaf, al-Ashjai is a well-known reliable narrator who has been mentioned many times before. The hadith has already appeared at 6745, narrated from Abdullah bin Amr, and the commentary there is lengthy. It should be added that Bukhari also recorded it, 6/269, no. 3166 (Fath). As did Abu Dawud, 3/83, no. 2760, Tirmidhi, 4/20, no. 1403, Nasai, 8/24, no. 4747, Ibn Majah, 2/896, no. 2686, and Darimi, 2/308, no. 2760.